iOS 4.0 im stuck

i was on 4.0 then i downgraded to 3.1,2 but i decided to upgrade back to 4.0
i backed up my shsh then every time i try upgrading i get this error....
'the ipod cannot be restored at this time because the ipod software update server could not be contacted or is temporarily unavailable'

You should go here:http://support.apple.com/kb/TS1275
and read the third bullet point near the bottom of the page entitled: Open the proper ports and allow access to Apple servers. Your problem may be that your computer has certain firewalls enabled that are restricting your access to Apple's servers.